Physical inactivity is related to many health problems including increased risk of cardiovascular disease, hypertension, and cancer. Despite the health consequences of a sedentary lifestyle, only 25% of Americans meet the recommended levels of physical activity. Researchers postulate that theory-based interventions effectively influence physical activity behavior by changing theoretical constructs believed to be important for behavior change (i.e., potential mediators), such as self-efficacy (i.e., confidence in one's ability to be physically active). Unfortunately, previous studies have generally failed to adequately test the mediating effect of theoretical constructs using appropriate statistical mediator analyses as recommended by leading behavioral scientists and statisticians. The objective of our application is to examine the mediating effect of theoretical constructs based on the Transtheoretical Model (TTM) and Social Cognitive Theory (SCT). Our proposed study will address the methodological, measurement, and statistical limitations of previous studies. We will randomly assign 420 healthy sedentary individuals to either a theory-based physical activity intervention or a contact control group. Physical activity behavior and the potential mediators will be assessed monthly over 12 months. We hypothesize that constructs based on The TTM and SCT (i.e., behavioral processes, cognitive processes, self-efficacy, and decisional balance) will mediate the relationship between group assignment (intervention vs. contact control) and physical activity behavior, such that the effect of the intervention on physical activity behavior relative to the contact control group will be attenuated when controlling for the mediators. We will also examine the importance of particular mediators for different population groups (e.g., age, gender), which will help to create more effective interventions for these groups. Finally, we will conduct a cost analysis to not only to inform policy makers of the cost of delivering the intervention within our study but also to estimate the cost of delivering each component of the intervention (i.e., mediator). Our analysis will estimate the cost of future refined interventions that incorporate those active components that affect the mediators and physical activity in this study. In summary, our proposed study will identify which mediators (i.e., theoretical constructs) are most effective for increasing physical activity, which will have an important public health impact given that this will likely lead to more efficacious and less costly interventions. [unreadable] [unreadable]